docs: def execute_route(self, meta_data, request_pdu):
          """ Execute configured route based on requests meta data and request
          PDU.
  
          :param meta_data: A dict with meta data. It must at least contain
              key 'unit_id'.
          :param request_pdu: A bytearray containing request PDU.
          :return: A bytearry containing reponse PDU.
          """
          try:
              function = create_function_from_request_pdu(request_pdu)
              results =\
                  function.execute(meta_data['unit_id'], self.route_map)
  
              try:
                  # ReadFunction's use results of callbacks to build response
                  # PDU...
                  return function.create_response_pdu(results)
              except TypeError:
                  # ...other functions don't.
                  return function.create_response_pdu()
          except ModbusError as e:
              function_code = get_function_code_from_request_pdu(request_pdu)
              return pack_exception_pdu(function_code, e.error_code)
          except Exception as e:
              log.exception('Could not handle request: {0}.'.format(e))
              function_code = get_function_code_from_request_pdu(request_pdu)
  
              return pack_exception_pdu(function_code,
                                        ServerDeviceFailureError.error_code)